It has long been recognized that the ability of a given loudspeaker transducer to efficiently reproduce low frequency sound in a typical listening space is largely a function of its enclosure size. The fundamental constraints regarding low frequency loudspeaker output were, in due course, correlated in general terms by the so-called Hoffman's Iron Law. First articulated in the 1960's, Hoffman's Iron Law correlates the low frequency output of a loudspeaker with certain physical parameters. These correlations were later expanded and refined by the engineers Thiele and Small whose work now serves as the preeminent mathematical modeling tool for the design of nearly all low frequency loudspeakers. Generally valid even today, Hoffman's Iron Law states that a speaker's acoustical efficiency is directly proportional to its enclosure volume and also the cube of its cutoff frequency. So, to decrease the lower cutoff frequency of a given loudspeaker by a factor of two, e.g. from 40 Hz to 20 Hz, while holding efficiency constant, an increase of enclosure volume must be made on the order of 23=8 times. And if it is desired to reproduce a 20 Hz signal without increasing enclosure volume, the loudspeaker requires 8 times more power to generate the same sound pressure level for a given motor efficiency. Forcing an acoustic transducer to operate incrementally below its natural resonant frequency requires exponentially more power. Generating significant low frequency sound from a small enclosure not only requires powerful amplifiers, but loudspeaker transducers that can effectively direct the large amount of resulting heat away from their voice-coils.
The vast majority of available high-power audio amplifiers, needed to drive presently offered compact subwoofers to sufficient sound pressure levels, utilize complicated circuit topographies with multiple gain stages. Such complex circuits require feedback loops to correct for their poor linearity. Amplifiers of this type are not generally favored by the high-end audio community. Very high-end vacuum-tube and solid state amplifiers, configured to operate in single-ended mode (a form of Class A operation) with little or no feedback, generate relatively small amounts of electrical power. Minimalist amplifiers of this sort are wholly incapable of driving to adequate listening levels currently available compact loudspeakers that are capable of very low frequency sound reproduction. Very large enclosures to are required to achieve reasonable sound pressure levels at very low frequencies with such amplifiers. When amplifier design limits power to just a few watts per channel (e.g. output-transformerless (OTL), single-ended designs utilizing triode vacuum tubes (SET)), very high efficiency bass horns are employed, which are not only enormous for a given low cutoff frequency, but very complicated, heavy, and expensive to manufacture and not without significant sonic shortcomings.
Subwoofers with small enclosure volumes are typically fitted with drivers with correspondingly small diaphragm areas. To help compensate for small surface areas, manufacturers have designed such transducers with very long stroke capabilities. However, because of their long stroke requirement, these drivers typically utilize motors incorporating over-hung voice-coils (i.e., the voice-coil height exceeds the height of its magnetic gap) to limit fabrication costs. This arrangement, however, leads to elevated levels of distortion and, due to the high induction of tall voice-coils, acoustical bandwidth is reduced and a time lag of the current passing through the voice coil occurs; compromising the driver's phase relationship with respect to sound being reproduced by other drivers in a system. The fabrication of a magnetic circuit with a gap height that sufficiently exceeds the height of the coil to accommodate the long excursions required, while achieving a high flux density, is prohibitively expensive.
The acoustical wave propagation impedance of a speaker cone of a given surface area is an inverse function of frequency squared. As such, a small area transducer with a long stroke does not impart an equivalent amount of low frequency acoustical energy into an ordinary listening space as a large area, short stroke driver with equivalent air displacements. Very large area acoustic diaphragms are, therefore, highly advantageous for low frequency performance; requiring a much shorter stroke for a given sound pressure level. A good wave propagation impedance match is particularly important for attenuators incorporating acoustic diaphragms which move passively. When coupled with a shallow, small volume enclosure of conventional design, however, the resonant frequency of a large area, lightweight acoustic diaphragm is very high; inhibiting low frequency performance. This is due to the comparatively large internal pressure change that occurs within such an enclosure for a given amount of acoustic diaphragm stroke.
Most low frequency loudspeakers utilize acoustic diaphragms that are moderately heavy and structurally rigid for the purpose of imparting pressure pulsations to air while undergoing relatively little physical deformation. While various shapes can be used, a conical structure is the most common with the cone driven from its apex via an attached voice-coil that is suspended within a cylindrical, annular gap of high magnetic flux. To achieve efficient operation, the magnetic strength of the motor assembly should be maximized, the voice-coil gap minimized, and the mass of the oscillating parts of the transducer minimized. Unless sophisticated construction techniques are utilized or exotic materials employed, however, thin-walled and lightweight acoustic diaphragms are usually too flexible, resulting in distortion and an uneven frequency response. Furthermore, over-damping of the excursions of large area and very lightweight acoustic diaphragms is difficult to prevent when coupled with large volume enclosures, limiting low frequency performance.
The fundamental purpose of a conventional loudspeaker enclosure is to prevent back waves, radiating from the rear of the acoustic diaphragm, from interacting adversely with the front waves. Speaker drivers coupled with conventional enclosures radiate as much energy into the enclosure as into the listening environment. Much of the energy directed into the enclosure, however, reappears in the listening space via the acoustic diaphragm which is ordinarily a poor sound barrier. This effect is most prevalent at low and middle frequencies where internal stuffing materials are less effective at absorbing sound. These two wave-fronts result in acoustic nulls and smeared transients when they interact in this manner. Although desirable for maximizing efficiency, thin-walled and lightweight acoustic diaphragms are highly susceptible to this problem.
By sufficiently delaying the low frequency back wave to be in phase, at steady-state conditions, with the front wave, some enclosure designs reinforce acoustical output at frequencies below resonance of the driver. Although low frequency efficiency is improved, this approach intrinsically results in poor impulse performance and significant acoustical phase shifts between the tuning frequency and other frequencies, degrading sound fidelity. Despite these shortcomings, a number of enclosure designs have been devised using variations of this approach, in order to maximize low frequency response for a given enclosure size. The more common examples include ported, passive radiator, band-pass, and transmission line designs.
Another type, the isobaric enclosure, is constructed of two (usually) identical drivers mounted to a relatively small sub-enclosure and wired in phase with the drivers mounted front-to-back in series. The back side of the rear driver sees the main enclosure volume, but the back side of the front driver sees the air volume of the sub-chamber between the two drivers. Being wired in phase, the pressure variations in the air space between the drivers is minimized thereby lowering the resonant frequency of the front driver. The front of the front driver faces the listening space. Isobaric loudspeakers are not very efficient, but the enclosure for a given low frequency cutoff point is fairly compact.
Operationally, the acoustic suspension enclosure is the simplest type; being comprised of a driver coupled with an enclosure of desired volume with air sealed within. This enclosure type is specifically designed to absorb the back wave, although in practice significant low and middle frequency sound passes back out through the speaker's acoustic diaphragm and into the listening space; interfering with the front wave and thus degrading sound fidelity. Since the back waves from the driver are deliberately absorbed, the acoustic suspension enclosure is not very efficient compared to some other enclosure types. The resonant frequency of the driver in this enclosure is always higher than its free-air resonant frequency because the enclosed air acts as a pneumatic spring coupled with it. One variation of the acoustic suspension speaker, referred to as an infinite baffle, utilizes an enclosure volume that is so large that the driver behaves nearly as it would if suspended in an unenclosed free-air space. However, if the acoustic diaphragm is particularly lightweight, its excursions are over-damped in this configuration, hindering low frequency output.
The driver in a typical acoustic suspension arrangement uses the volume of air contained within its enclosure to act as a spring coupled with the acoustic diaphragm. For a given excursion of the acoustic diaphragm, the pressure change within the enclosure can be reduced by increasing its volume. A large enclosure volume thus mimics a weak spring giving the acoustic diaphragm a low in situ resonant frequency. Ideally, the resonant frequency of the woofer is below the lower frequency limit of human hearing, i.e. approximately 20 Hz, or even lower if visceral sounds, such as earthquakes, thunder, helicopters, etc., are to be reproduced. Such a low resonant frequency with a typical acoustic suspension arrangement can be achieved with either a heavy acoustic diaphragm coupled with a small enclosure or a lightweight diaphragm coupled with a large enclosure. Heavy acoustic diaphragms hurt efficiency and an enclosure providing for a very low resonant frequency of a driver of normal mass must be very large. The typical enclosure volume required for an acoustic suspension design incorporating an ordinary 15 inch driver of normal mass operating flat to 20 Hz is approximately 7 ft3. Such a large enclosure is prone to sympathetic low frequency resonances within its structure which are audible and thus problematic for accurate sound reproduction. The walls of large enclosures must ordinarily be braced or otherwise increased in thickness to inhibit such resonances.
Compared to designs that use the back waves of the driver to augment the output of the front waves, the phase shift between the lower and upper frequencies is not as severe with acoustic suspension loudspeakers, but it is significant nonetheless. Low frequency drivers operating in typical acoustic suspension enclosures nearly always have resonant frequencies in the audible range, i.e., 20 Hz or above. Proximate their resonant frequency points, sounds emitted by low frequency drivers experience a large phase shift. To preserve phase coherency, some manufacturers operate their low frequency drivers below resonance. But since acoustic output from a driver diminishes rapidly below resonance, significant amplifier compensation is required. This design approach, therefore, requires a very powerful amplifier for strong low frequency performance. Compared to designs that use the back waves of the driver to augment the output of the front waves, the phase shift between the lower and upper frequencies is not as severe
A very recent innovation described in U.S. Pat. No. 7,068,806 utilizes chambers of varied pressures acting on differing surface areas to reduce the resonant frequency of the acoustic diaphragm of a low frequency loudspeaker. This design preserves the performance advantages of a standard acoustic suspension speaker and can achieve a low resonant frequency of its acoustic diaphragm. But it must use very high pressures acting on a very small surface area to achieve this with a compact enclosure. Although the enclosure size is greatly reduced for a given resonant frequency, there is still a correlation between enclosure volume and resonant frequency.
The motor assembly of a speaker transducer, which is responsible for converting electrical energy to acoustical energy, must be designed to dissipate heat from its voice-coil. Many speakers employ ferrofluids to limit voice-coil temperature during operation. Ferrofluid is a thermally conductive ferritic liquid used to fill the magnetic gap within which the voice-coil is suspended; conducting heat away from the voice-coil. Ferrofluid is retained in the gap by the magnetic flux, but its retention is difficult in drivers with large excursion capabilities. Furthermore, the viscosity of ferrofluid is prone to change over time, especially in severe service applications, which changes the operating characteristics of the driver. Woofers that do not use ferrofluids must ordinarily incorporate a ventilation system into the motor assembly for sustained operation at high sound pressure levels. Either way, the heat liberated from the motor assembly is ordinarily trapped within acoustic suspension enclosures since, by design, they are airtight. Furthermore, internal enclosure stuffing, needed to absorb the internal sound waves, greatly inhibits heat transfer from the enclosure.
In the field of sound attenuation, acoustic blankets and active feedback motion control attenuators are the common means of dissipating or canceling undesirable very low frequency acoustic energy that may exist in confined spaces. Active devices have the disadvantages of being bulky, heavy and requiring an amplifier and servo control circuitry. Acoustic blankets are heavy and not particularly effective at attenuating the very lowest frequencies. Currently existing passively moving acoustic diaphragm attenuators are also heavy and bulky and have limited attenuation effectiveness and operate over a narrow bandwidth. The need for effective, lightweight and very low frequency attenuators is particularly acute within launch vehicle shrouds where sensitive electronics and structurally fragile payloads are exposed to extremely intense sound pressure levels; much of it at infrasonic frequencies. When the aforementioned conventional means of sound mitigation are not used, an increase in the robustness of the payload structures must be made. Each of these solutions, however, increases launch weight; resulting in reduced payload capacity.